<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css">
			.a:before{
				content: '我是通过content添加的';
			}
			:empty{
				width: 100px;
				height: 100px;
				border: 1px solid;
			}
		</style>
	</head>
	<body>
		<div class="a"></div>
		<div>我是有内容的</div>
	</body>
</html>
